1. Field of the Invention
The present invention relates generally to optical rare-earth doped fibers with small clad diameters and, more specifically, to coiled or bent optical rare-earth doped fibers with small clad diameters.
2. Technical Background
Rare earth doped optical fibers are commonly utilized in optical amplifiers. An example of optical amplifier is an erbium doped optical fiber amplifier (EDFA). A typical erbium doped fiber has an outer clad diameter of 125 μm. When this type of fiber is coiled such that bend radius is small (35 mm or smaller), the fiber coils suffer from bend-induced birefringence. The tighter (smaller) the bend radius, the higher the amount of bend-induced birefringence. More specifically, the bend-induced birefringence is inversely proportional to the square of the bend radius. The bend induced birefringence results in Differential Group Delay or DGD. As illustrated in FIG. 1, the smaller the bend radius, the greater the DGD. That is, due to bend-induced birefringence one polarization component of the optical signal propagates through the fiber faster than the other polarization component. As a result, because Polarization Mode Dispersion or PMD is the average of DGD values over the wavelength band, the bend induced birefringence also results in increased PMD. This is especially problematic in high data rate amplifiers because PMD broadens the signal pulse width, thereby limiting the error-free bit rate of a fiber optic transmission system. Thus, DGD and PMD are limiting factors that prevent utilization of smaller coil diameters in high data rate optical amplifiers.